Generally speaking, male rats make better "lap" pets, preferring to sit and have their ears scratched by an attentive human friend. Female rats are very curious, and love to explore and play games. Both genders make great companions. I recommend getting a pair of males *or* females (they are very social creatures and really like to have a cagemate of the same gender!) because they both are absolutely charming. I would suggest you *let a rat pick you* when you go pick them up, and then get a friend of the same gender and age (a littermate would be great if there's one available), so your new rattie won't be lonely when you aren't around to play.
If these are your first rat(s), I would recommend scanning through some of the articles on Virginia's Rat Page:
as you can find more information on your new pet(s) as you need it.